Heavy-Duty Truck

Background

Trucks are divided into light-duty, medium-duty, and heavy-duty
classifications depending on their weight. Heavy-duty trucks have a gross
vehicle weight of 33,000 lb (15,000 kg) or more (i.e. the weight of the
vehicle plus the weight of the payload is 33,000 pounds or more). When a
heavy-duty truck is pulling a trailer, it may have a gross combination
weight of 80,000 lb (36,360 kg) or more.

Technically, a vehicle that carries the load by itself, without a trailer,
is known as a truck, or a straight truck. Examples include certain dump
trucks, concrete mixers, and garbage trucks. A vehicle that pulls the load
in a trailer is known as a tractor. The tractor is coupled to the trailer
through a pivot point, known as the fifth wheel, which is mounted on top
of the tractor frame. Most of the big rigs on highways are tractors
pulling trailers.

History

The first gasoline-engine trucks were developed in the United States in
the 1890s. During World War I, trucks played an important role moving
supplies at home and overseas. With the development of a system of paved
roads in the United States during the 1920s, the number of truck
manufacturers grew. By 1925, there were more than 300 brands of trucks on
the road. Some manufacturers came and went quickly. The Great Depression
of the 1930s finished many more. By the 1990s, there were only nine
heavy-duty truck manufacturers left in the United States. Together they
build about 150,000-200,000 trucks a year.

Raw Materials

Trucks use steel for strength and durability, aluminum for light weight
and corrosion resistance, polished stainless steel for bright finishes,
and molded plastics for complex shapes.

Frame rails and crossmembers are usually formed from high-tensile steel.
Suspension components, axles, and engine mounts are also made from steel.
Some are cast and some are fabricated and welded.

The cab structure and outer skin may be made from steel or aluminum. If
steel is used, the metal is coated with one or more layers of corrosion
barriers such as zinc. On some cabs the roof may be made of fiber-glass to
form the complex curves required at the corners.

The hood and front fenders are usually molded in plastic or fiberglass
because of the complex aerodynamic shapes. The front bumper may be stamped
and drawn from steel or aluminum, or it may be molded in plastic and
backed with a steel substructure.

Bright trim pieces—such as outside mirrors, sun visors, radiator
grilles, and grab handles—are often made from polished stainless
steel to give a long-lasting bright finish that will not crack or corrode.

The cab interior is finished with vinyl or cloth upholstery. The floors
are covered with synthetic fiber carpeting or rubber mats. The dashboard
and interior trim pieces are molded from plastic. The windows are made of
laminated safety glass.

Fluids used in heavy-duty trucks include diesel fuel, petroleum-based or
synthetic lubricants,
antifreeze, power steering fluid, and an environmentally safe,
non-fluorocarbon gas known as R134A, which replaces freon in the air
conditioning system.

Design

Truck manufacturers usually design a new model about every five to seven
years. The new design incorporates advances in technology and materials,
as well as changes desired by the customers. The design team will use a
clay model to determine the overall styling, then build a prototype cab
and hood for review and durability testing. As the design progresses, they
will build an entire prototype vehicle for road testing. Just before the
new truck goes into production, they will build one or more pilot models
using actual production parts to spot any last-minute assembly problems.

In addition to the basic model, the engineers must also design all the
options required by customers for different truck applications. Some
manufacturers have as many as 12,000 options for their line of heavy-duty
truck models.

The Manufacturing
Process

Heavy-duty trucks are assembled from component parts. Each truck
manufacturer usually builds its own cabs, and a few also build their own
engines, transmissions, axles, and other major components. In most cases,
however, the major components (and many of the other components) are built
by other companies and are shipped to the truck assembly plant.

In most plants, the trucks move along an assembly line as components are
added by different groups of workers at successive workstations. The truck
starts with a frame assembly that acts as the "backbone" of
the truck and finishes with the completed, fully operational vehicle being
driven off the end of the assembly line under its own power.

Today's most recognized form of heavy-duty truck, the
tractor-trailer, or semi truck, was commercially developed in the 1910s.
Some truck designers believed tractors, motor trucks designed only to
pull separable trailers, could make truck operation profitable. If
tractors easily connected to trailers, the more costly motorized
tractors could remain busy hauling full trailers, while leaving less
expensive trailers idle during loading or unloading.

In 1911, truck designer Charles Martin built a gasoline powered tractor
to pull modified horse-drawn commercial wagons. His most significant
innovation, however, was the fifth wheel coupler. A round plate with a
central hole, it attached to the top of tractor frames to connect and
support trailers. Buyers converted wagons into semi-trailers by raising
them with jacks and removing their front axles. Lowering and locking a
trailer's bottom mounted kingpin into a tractor's fifth
wheel coupled the vehicles. Martin's Rocking Fifth Wheel handled
the period's rough roads. It allowed tractor-trailers to bend
when turning, but also accommodated the ups and downs of uneven
surfaces. Nearly every truck manufacturer purchased Martin's
popular device.

August Fruehauf, a Detroit blacksmith, launched an early trailer
manufacturing company in 1914, by building a boat trailer for local
lumberman Frederick Sibley. Sibley pulled it with a Model-T Ford car
that he turned into a one ton truck with a Smith Form-A Truck conversion
kit. Impressed that pivoting tractor-trailers maneuvered long, heavy
loads through tight quarters, Sibley ordered more trailers for his
business. By 1916, Fruehauf was a noted trailer manufacturer.

Erik R. Manthey

Here is a typical sequence of operation for the assembly of a heavy-duty
truck:

Assembling the frame

1 A pair of frame rails are selected from stock lengths of C-channel.
They are laid side-by-side and fed through an automatic drilling machine
or punch to make holes for connecting crossmember brackets, engine
mounts, and other frame-mounted components. A computer tells the machine
the size and location of the required holes along the length of the
frame rails.

2 Small threaded studs are spot welded inside the C-section of the frame
rails. The air lines for the brakes and the electrical wires for the
lights and sensors are placed inside the frame rails and are secured
with rubber-cushioned clamps fastened to the studs.

3 The brackets for the frame crossmembers are bolted in place using
high-strength bolts or self-clinching fasteners. The left and right
frame rails are then positioned opposite each other, and the
cross-members are added. The frame now resembles a long ladder with the
rails as the sides and the crossmembers as the rungs.

4 Other frame-mounted components—such as engine mounts,
suspension brackets, and air tanks—are bolted in place.

Installing the axles and suspensions

5 The front and rear axles are fitted with the proper hubs (the round
ends to which the wheels are attached), brakes, and brake drums. The
axles are clamped to the suspensions by means of long u-bolts. Some
suspensions use long leaf springs while others use inflated rubber air
bags.

6 The front and rear axles and suspensions are lifted into place and
attached to the suspension brackets on the frame. The shock absorbers
are attached between the axles and the frame.

Finishing the frame

7 Up until this point the frame assembly is usually moved from station
to station either manually or with overhead hoists. The frame is now
placed on a moveable support and begins moving down the assembly line.
The air tanks and brake chambers are connected to the air lines, and the
lights and sensors are connected to the proper wires.

8 If the vehicle is to be a tractor, the fifth wheel is lifted onto the
frame and bolted into place. From this point on the frame assembly with
the axles, suspensions, and frame-mounted components is referred to as
the chassis.

Painting the chassis

9 All components that are not to be painted are covered with masking
tape or paper. The chassis then moves into a paint booth where it is
painted with compressed air spray guns. Most truck manufacturers require
that all component parts be received with a primer coat of paint, so
priming is not necessary.

10 After the chassis has been thoroughly painted and visually checked,
it moves into a drying oven where a flow of hot air dries the paint. As
it emerges from the oven, the masking tape and paper are removed.

Installing the engine and transmission

11 The engine and transmission are brought into the plant alongside the
assembly line. Almost all trucks now use diesel engines. The clutch is
installed and the transmission is bolted onto the rear of the engine.
The fan, altemator, and other engine components are installed and
connected with hoses and electrical wiring.

12 The finished engine/transmission package is then hoisted using
lifting eyes that are part of the engine and is lowered onto the engine
mounts in the chassis, where it is bolted in place. The radiator
assembly is bolted onto its brackets ahead of the engine. The fuel
lines, air hoses, starter cables, and coolant hoses are connected to the
engine.

Finishing the chassis

13 The fuel tanks are secured to their frame brackets and connected to
the fuel lines. Batteries are secured in the battery box, but are not
connected to prevent accidental sparking.

14 The tires are mounted on the wheels at a workstation adjacent to the
assembly

line. Aluminum wheels are left natural or may be polished. Steel wheels
are painted before the tires are mounted. The tires and wheels are
mounted on the axle hubs, and the lug nuts are tightened. At this point,
the truck is taken off its moveable supports and sits on its own tires.

Assembling the cob, hood, and sleeper

[Steps 15-23 are performed in a separate area off the assembly line]

15 The cab and sleeper substructures are welded or fastened together in
jigs to hold the pieces in place. The substructures give the cab and
sleeper their strength and provides fastening points for the outer skin
and the inner upholstery and trim.

16 The outer skin pieces are welded or fastened in place. This includes
the sides, back, floor, and roof pieces. The joints between pieces are
overlapped and sealed to prevent leaks. The cab and sleeper doors are
secured to the hinges.

17 The hood is usually a molded plastic piece and is shipped to the
plant without any hardware attached. The hood is checked for rough
surfaces and is sanded as required.

Painting the cab, hood, and sleeper

18 The cab, hood, and sleeper for each truck are painted at the same
time. The surfaces are cleaned and the areas that are not to be
painted are masked off with paper or tape. If a paint design such as a
different color stripe is specified, the stripe area is painted first,
then the stripe is masked off and the main body color is applied on a
second pass through the paint booth. After each pass, the cab, hood,
and sleeper go through a drying oven. After the final pass, the
masking is removed and the paint is visually inspected.

In most plants, the trucks move along an assembly line as
components are added by different groups of workers at successive
workstations. The truck starts with a frame assembly that acts as
the "backbone" of the truck and finishes with the
completed, fully operational vehicle being driven off the end of
the assembly line under its own power.

Finishing the cab, hood, and sleeper

19 The grille, headlight brackets, hood hinges and latches, and the
manufacturer's emblem or name are installed on the hood. The
finished hood is then stored alongside the assembly line.

20 The exterior components of the cab and sleeper—the grab
handles, mirrors, visors, etc.—are mounted before any work on the
interior begins.

21 The instrument panel is attached to the dashboard. The gauges,
warning lights, and switches are installed and hooked up to the
appropriate wires and hoses. The entire dashboard assembly is then
installed in the cab along with the cab heater system and steering
column.

22 Pads of foam insulation are placed in the cab and sleeper walls, and
the interior upholstery pieces are secured in place on the walls and
ceiling. Plastic trim pieces are screwed in place to cover exposed edges
and seams. The floor is covered with a rubber mat or fabric carpet
laminated to a sound-absorbing pad, and the edges are secured. The seats
are installed on top of the floor covering and secured with bolts into
the main cab structure.

23 The windshield and rear windows are carefully pressed into place. A
rubber gasket seals the edges between the glass and the cab structure.

Installing the cab, hood, and sleeper

24 The completed cab is lowered onto the chassis and bolted to its
mounts. The sleeper is bolted in place behind the cab. The steering
column is connected to the steering box. The transmission shift lever is
installed through the floorboard, and the clutch pedal is attached to
the clutch linkage.

25 After all the cab connections are made, the hood is lowered onto the
chassis and secured to its pivot point. The bumper is attached to
brackets on the frame. Wire connections are made for the headlights and
front turn signals.

Adding fluids

26 The engine, radiator, and other reservoirs are filled, and the air
conditioning system is charged. A small amount of diesel fuel is added
to the tanks to allow a short road test. The steering wheel, which had
been left out to give working room in the cab, is now installed, and the
batteries are connected. The completed truck is then driven off the end
of the assembly line.

Aligning the front and rear axles

27 To make sure that the front and rear axles are parallel to each other
and perpendicular to the centerline of the frame, the truck is placed on
a laser alignment machine and the axle positions are adjusted as
required. The angle of the wheels is also adjusted. This ensures that
the truck will handle properly and have satisfactory tire life.

Testing the completed truck

28 The truck is driven onto a dynamometer and secured with chains. The
rear wheels of the truck sit on rollers set into the ground and
connected to the dynamometer. As the truck engine spins the rear wheels
on the rollers, the dynamometer measures the engine power to ensure it
is operating correctly.

29 The truck is driven slowly through a water spray booth as the driver
checks for cab leaks. The driver then takes the truck out for a short
drive to check out the overall operation. If the truck passes all the
tests, it is parked on "ready row" to be delivered to the
dealer.

Quality Control

In addition to testing the completed truck, each component part and
assembly operation is inspected. Parts are checked for correct dimensions
before they reach the assembly line. Assembly operations are checked by
the production workers themselves and are double-checked by quality
control inspectors. The instrument panel is tested to make sure all the
gauges and switches are working before it is installed in the truck. Even
the thickness of the paint is checked with an electronic meter to ensure
it meets the standard.

The Future

Heavy-duty trucks have evolved slowly over the last 100 years and will
probably continue a slow evolution in the future. An increased concern
about fuel efficiency has led to more aerodynamic designs. Likewise an
increased concern about exhaust emissions has led to cleaner combustion
engines. Heavy-duty trucks are still one of the most economical ways to
ship the wide variety of raw materials and finished goods needed in our
complex society, and they will probably remain one of our principal forms
of transportation for many decades to come.

Where to Learn More

Books

Karolevitz, Robert F.
This Was Trucking.
Superior Publishing Company, 1966.

User Contributions:

Hey,
Saftety suggestion from a non-trucker:
How about a high mounted running / brake light attached to the center-back of the roof of the cab similar to the high-mounted brake light on cars or vans. I was driving the highway in a rainstorm behind a truck without a trailer and could not detect the rear running / brake lights on the truck with all the road spray...dangerous. See and be seen.
sincerely,
Kit Greaves
Bowmanville, Ontario, Canada

Comment about this article, ask questions, or add new information about this topic: